1. Technical Field
The present disclosure generally relates to a flow meter, and more particularly to a thermal mass flow meter for measuring fluid velocity.
2. Description of Related Art
A thermal mass flow meter is usually used to measure the fluid velocity in the pipeline, thus judging whether the fluid flows or not. In general, multiple probes of the thermal mass flow meter are arranged in the pipeline when the thermal mass flow meter is assembled to the pipeline. In particular, two resistors and two thermal resistors are electrically connected to the Wheatstone bridge circuit. Also, the two thermal resistors are installed in one of the probes and the two resistors are installed outside the thermal mass flow meter, and a heating module is installed in another of the probes.
The thermal mass flow meter is used to measure the fluid velocity in the pipeline according to the relationship of the fluid velocity is inversely proportional to the temperature difference. Also, the thermal mass flow meter provides a control unit to control heating units installed in the probes to heat the fluid in the pipeline. When the Wheatstone bridge circuit is balanced because of the unchanged resistance values, there is no current through the control unit. That is, it indicates that the fluid velocity of the fluid is normal. However, the corresponding resistance values of the two thermal resistors would be changed to cause the current variation when the increased temperature by heating the heating unit is stagnated around the probes. After the current variation is transmitted to the control unit, the fluid velocity of the fluid can be measured according to the sensed temperature values because the fluid velocity is inversely proportional to the temperature.
Although the thermal mass flow meter can be used to measure the fluid velocity of the fluid in the pipeline, the measured results from the two resistors installed outside the thermal mass flow meter would be not completely correct due to the influence of the external ambient temperature.